(1) Field of the Invention
The present invention relates to an organic electroluminescence device having excellent luminescence properties.
(2) Description of the Prior Art
An organic electroluminescence device (hereinafter abbreviated to the organic EL device) is a light-emitting device which makes use of the principle that when an electric field is applied, a fluorescent material emits light in response to the charge recombination of holes injected from an anode and electrons injected from a cathode. After C. W. Tang et al. of Eastman Kodak Company reported a low-voltage-driven organic EL device using a double layered structure (C. W. Tang, S. A. Vanslyke, Applied-Physics Letters, 51, 913(1987) and the like), studies on an organic EL device have been briskly carried out. Tang et al. reported an organic EL device using tris(8-hydroxyquinolinol aluminum) in an electron-transporting and light-emitting layer and a triphenyldiamine derivative in a hole-transporting layer. This stacked structure gives such advantages as an improvement in the injection efficiency of holes into the light-emitting layer; blocking of electrons injected from a cathode, which increase the efficiency of exciton production from charge recombination; and confinement of the excitons into the light-emitting layer. A double layered structure composed of a hole-injecting and transporting layer and an electron-transporting and light-emitting layer or a triple layered structure composed of a hole-injecting and transporting layer, an electron-transporting and light-emitting layer and an electron-injecting and transporting layer is well known as an organic EL device. In order to increase the recombination efficiency of injected holes and electrons, various improvements in the device structure or fabrication process have been introduced to such multi-layered devices.
As a hole-transporting material, triphenylamine derivatives such as 4,4′,4″-tris(3-methylphenylphenylamino)-triphenylamine and aromatic diamine derivatives such as N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1-biphenyl]-4,4′-diamine, which are starburst molecules, are well known (e.g., Japanese Patent Application Laid-Open Nos. 20771/1996, 40995/1996, 40997/1996, 543397/1996 and 87122/1996).
As an electron-transporting material, oxadiazole derivatives, triazole derivatives and the like are well known.
As an electron-transporting and light-emitting material, chelate complexes such as tris(8-quinolinolate)aluminum complex are known, and it is also well-known that devices using the mixture of a light-emitting material and a small amount of a fluorescent pigment have an improved efficiency. As these fluorescent pigments, there are known coumarin derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives, oxadiazole derivatives, porphyrin derivatives, phenoxazoline type pigments, rhodamine type pigments, acridine type pigments and the like. It is reported that various color light in a visible region from blue to red are obtained from these materials, and realization of a full color organic EL device is expected (e.g., Japanese Patent Application Laid-Open Nos. 239655/1996, 138561/1995, 200289/1991 and the like).
Nowadays, the organic EL devices having a high luminance and a long life have been reported, but they are not always sufficient. Therefore, it has be strongly desired to develop the materials having a high performance.